intersect two lists with different objects

Question

I have a list of ObjA and ObjB as follows:

List List1;
List List2;

Both ObjA and ObjB has a common field which is U

Answer 1

1.look at this simple code

  var result = (from objA in objAList
                join objB in objBList on objA.user.Id equals objB.user.Id
                select objA/*or objB*/).ToList();

2.complete code

 class QueryJoin
{
    static void Main(string[] args)
    {
        //create users
        User user1 = new User { Id = 1, Name = "anuo1" };
        User user2 = new User { Id = 2, Name = "anuo2" };
        User user3 = new User { Id = 3, Name = "anuo3" };
        User user4 = new User { Id = 4, Name = "anuo4" };
        User user5 = new User { Id = 5, Name = "anuo5" };
        //create objAList
        List<ObjA> objAList = new List<ObjA>();
        objAList.Add(new ObjA { user = user1 });
        objAList.Add(new ObjA { user = user2 });
        objAList.Add(new ObjA { user = user3 });
        //create objBList
        List<ObjB> objBList = new List<ObjB>();
        objBList.Add(new ObjB { user = user3 });
        objBList.Add(new ObjB { user = user4 });
        objBList.Add(new ObjB { user = user5 });

        //intersect
        var result = (from objA in objAList
                      join objB in objBList on objA.user.Id equals objB.user.Id
                      select objA/*or objB*/).ToList();

    }

}

class ObjA
{
    public User user { get; set; }
}

class ObjB
{
    public User user { get; set; }
}

class User
{
    public int Id { get; set; }
    public string Name { get; set; }
}

Answer 2

without need of IEqualityComparer or IEquatable (which would be better anyway)

var commonUsers = list1
                  .Select(l1 => l1.User)
                  .Where(u => list1
                       .Select(l => l.User.Id)
                       .Intersect(list2
                          .Select(l2 => l2.Id))
                       .Contains(u.Id));

or

var commonUsers = list1.Select(l1 => l1.User)
                      .Where(u=> list2.Select(l2 => l2.User.Id)
                                        .Contains(u.Id));

Answer 3

The general idea is

var commonUsers = list1.Select(a => a.User).Intersect(list2.Select(b => b.User));

However, by itself this assumes that User implements IEquatable<User>, which does not seem to be the case here. So you either need to add this implementation or use the Intersect overload that accepts a custom IEqualityComparer<User>.

Answer 4

The standard way is to utilize an IEqualityComparer object. The default one uses the standard equality comparison. Create a class that implements the IEqualityComparer interface and performs the comparison you want. Then you can call an overload of IEnumerable.Intersect that accepts an instance of your custom compare class